This was posted in the "Ball Earth Conundrums" thread, for which it was off topic, so I am posting it here.
Some background: a FE proponent was asked how fast the sun travels in the FE model, and his answer described teh sun traveling in a circle, the diameter of which is equal to the diameter of the earth. However... his answer is incorrect, because in the (North-centric) flat earth model, the sun would not travel at the extremity of the earth's diameter, but rather within a washer-shaped plane where the inner circle is the Tropic of Cancer and the outer circle is the Tropic of Capricorn. The outer radius of this circle is approximately 12, 600 kilometres (distance from North pole to T. of Cap), so the circumference would be ~79,100 Km. The speed of the sun at the December solstice would therefore be about 3300 Km/hr, and would vary downwards to the June solstice where it would be about 1930 Km/hr.
Here's the bigger problem with all that...
This FE model violates both the first law of thermodynamics and the law of conservation of angular momentum. It violates the former because it requires a change of kinetic energy to change direction at the solstices, and the latter because with a decreasing radius and a constant mass, the sun would have to increase in velocity as the radius decreased. Further, there is no mechanism provided (FE proponents reject gravity) to explain the changes in velocity or, for that matter, of direction.
Again, the North-centric flat earth model is physically impossible.